Large ladies and parisitic males: extreme sex in the deep sea

Distributed throughout the world’s oceans, at depths greater then 300m, this dark, vast and often food-scarce environment is home to a suborder of anglerfish known as the Ceratioidei, that exhibit a very unique form of reproduction.

They are the most species-rich vertebrate group within the bathypelagic zone (1000 to 4000 metres deep) with 162 recognised species from 11 families. Commonly known for their large heads, fang-like teeth and distinctive bioluminescent lure – hanging over the mouth like a fishing rod to draw in prey from the immense darkness (fig 1.). However, these features are only observed from females and, for a while, the only Ceratioids observed were the large females. This left scientists with a mystery as to where all the males were located. As exploration into the deep sea progressed, the males were still not found and the enigma surrounding Ceratioid reproduction continued. It was not until almost a century after the first ceratioid was catalogued that a male was discovered, in a most unexpected spot, fused to the side of a female – living off her as a parasite.

“This remains a puzzle for some future researcher to solve”

Seamundsson in 1922, wondering why, what he thought to be young anglerfish (later discovered to be males) were attached to the female.

Fig. 2 An 11mm parasitic male attached to the larger 77mm female of the species Bertella idiomorpha. Source

“It would be very difficult for a mature fish to find a mate, but this difficulty appears to some extent to have been got over by the males”

This phenomenon has drawn some attention due to its unusual and interesting nature, however, sexual parasitism has not been found in all species of the Ceratioids. Currently sexual parasitism has been found in 24 species comprising 6 families. Other species are thought by scientists to possibly have free-living males that are able to feed and live independently of the female, nipping onto her side to reproduce but not fully fusing. The lack of data on many families and species makes it difficult to determine how common sexual parasitism is within this suborder; for many families the possibility has not been ruled out.

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Extreme marine habitats is a site dedicated to those habitats that are simply extreme! Written by students, we aim to provide detailed and insightful information on a variety of marine science subjects.